The present invention relates to sequence classification such as required when carrying out machine translation of natural language sentences.
In machine translation, the objective is to translate a source sentence such as the English sentence                I need to make a collect callinto a target sentence, such as the Japanese version of that sentence This task is a special case of the more general problem known as sequence classification.        
Stated in more general terms, the natural language translation problem can be understood as a specific case of taking a source symbol sequence and classifying it as being a particular target symbol sequence. For convenience, the discussion herein uses the terms “word,” “sentence,” and “translation” rather than “symbol,” “sequence” and “classification,” respectively. It is to be understood, however, that the invention is applicable to the more general case of translating one sequence of symbols into another. It will also be appreciated that the invention is applicable not only to grammatically complete sentences but to phrases or other strings of words that amount to something less than a complete grammatical sentence, and thus the word “sentence” in the specification and claims hereof is hereby defined to include such phrases or word strings.
The task of identifying the target sentence word that corresponds to a source sentence word would be somewhat straightforward if each source language word invariably translated into a particular target language word and all in the same order. However, that is often not the case. For example, the English word “collect” in the above sentence refers to a type of telephone call in which the called party will be responsible for the call charges. That particular meaning of the word “collect” translates to a particular word in Japanese. But the word “collect” has several other meanings, as in the phases “collect your papers and go home,” and “collect yourself, you're getting too emotionally involved.” Each of those meanings of the word “collect” has a different Japanese language counterpart. And word order varies from one language to the next.
The probability that a particular word in the target vocabulary is the correct translation of a word in the source sentence depends not only on the source word itself, but the surrounding contextual information. Thus the appearance of the word “call” directly after the word “collect” in an English sentence enhances the probability that the Japanese word  is the correct translation of the word “collect” because the use of the two words “collect” and “call” in one English sentence increases the probability that “collect” is being used in the source sentence in the telephone context.